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 Table of Contents  
REVIEW ARTICLE
Year : 2017  |  Volume : 2  |  Issue : 1  |  Page : 14-17

Epigenetic programming of autonomic functions in an experimental model of apnea of prematurity


Institute for Integrative Physiology and Center for Systems Biology of O2Sensing, Biological Science Division, University of Chicago, Chicago, USA

Date of Submission11-Apr-2017
Date of Acceptance18-Apr-2017
Date of Web Publication1-Jun-2017

Correspondence Address:
Nanduri R Prabhakar
Institute for Integrative Physiology, Biological Science Division, 5841 S. Maryland Avenue, MC 5068, Room No. 711, Chicago, IL 60637
USA
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/bjhs.bjhs_8_17

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  Abstract 

Intermittent hypoxia (IH) is a hallmark manifestation of recurrent apneas, which is a major clinical problem in infants born preterm. Carotid body (CB) chemoreflex and catecholamine (CA) secretion from adrenal medullary chromaffin cells (AMCs) are two major mechanisms contributing to the maintenance of cardiorespiratory homeostasis during hypoxia. The purpose of this article is to highlight recent studies showing how neonates experiencing IH affect the CB and AMC function and their consequences in adult life. To simulate apneas, rat pups were treated with IH consisting of alternating cycles of hypoxia (1.5% O2) for 15 s and room air for 5 min, 8 h/day from ages P0–P10. Rats treated neonatal IH displayed augmented CB response to hypoxia and augmented CA secretion from AMC. Rats treated for 10 days of IH in the neonatal period were allowed to grow into adulthood. Remarkably, the effects of neonatal IH on CB and AMC persisted in the adulthood. Moreover, adult rats that were exposed to IH in neonatal period exhibited hypertension, increased incidence apnea. Analysis of the underlying molecular mechanisms revealed re-programming of the redox state by epigenetic mechanisms involving suppression of transcription of antioxidant enzyme genes by DNA hypermethylation. DNA hypomethylating agents might offer a novel therapeutic intervention to prevent early onset of cardiorespiratory morbidities caused by neonatal IH.

Keywords: Apnea of prematurity, cardiorespiratory morbidities, DNA methylation, oxidative stress


How to cite this article:
Nanduri J, Prabhakar NR. Epigenetic programming of autonomic functions in an experimental model of apnea of prematurity. BLDE Univ J Health Sci 2017;2:14-7

How to cite this URL:
Nanduri J, Prabhakar NR. Epigenetic programming of autonomic functions in an experimental model of apnea of prematurity. BLDE Univ J Health Sci [serial online] 2017 [cited 2017 Aug 22];2:14-7. Available from: http://www.bldeujournalhs.in/text.asp?2017/2/1/14/207430

Infants born preterm often exhibit recurrent apnea (brief, repeated cessation of breathing) resulting in intermittent hypoxia (IH).[1] Cardiorespiratory responses to hypoxia depend on reflexes arising from the carotid body (CB), the primary sensory organ for monitoring arterial blood O2 levels. Carotid bodies, however, are immature at birth and show maturation of O2 sensing during the 1st week of neonatal life.[2],[3],[4] Too little (hypoxia) or too much environmental O2(hyperoxia) during neonatal life profoundly impacts maturation of CB O2 sensing.[3],[4] Catecholamine (CA) secretion from adrenal medullary chromaffin cells (AMCs) is another important mechanism for maintaining cardiovascular homeostasis under hypoxia.[5],[6] In neonates, sympathetic innervation to the target organs is incomplete,[5],[6] and hypoxia facilitates CA secretion by directly affecting the excitability of AMC.[7],[8] Infants with recurrent apnea exhibit autonomic dysfunctions including (a) altered sympathoadrenal function [9] (b) augmented ventilatory response to hypoxia, a hallmark of CB chemoreflex,[10] and (c) cardiac arrhythmias.[11] Recent studies on rodent models have shown that neonatal IH profoundly affects the CB O2 sensing and CA secretion from AMC. In this article, we provide a brief review of studies addressing the mechanisms underlying the effects of neonatal IH on the CB and AMC and their impact in adult life.


  Effects of Neonatal Intermittent Hypoxia on Hypoxic Sensing by the Carotid Body Top


Carotid bodies from neonatal rat pups respond poorly to hypoxia.[3],[4],[12] Neonatal rats exposed to chronic hypoxia exhibit reduced CB response to hypoxia.[13],[14] In striking contrast, rat pups exposed to IH from ages P0 to P10 (15 s of hypoxia followed by 5 min of normoxia, 9 episodes/h, 8 h/day) exhibit augmented CB response to hypoxia.[12],[15],[16] The augmented sensory response to hypoxia could be seen in ex vivo carotid bodies, suggesting that this response is independent of circulatory changes.[12],[15],[16] Although IH leads to a similar augmentation of the CB response to hypoxia in adult rats, there are some notable differences between the effects of IH in neonatal versus adult carotid bodies.[12],[17] First, the augmented hypoxic sensitivity in neonates can be seen with exposures to as little as 72 episodes of IH, whereas adult rats require as many as 720 IH episodes, suggesting that neonates are relatively more sensitive to IH than adults.[15] Second, in IH-exposed adult rats, repetitive hypoxia leads to long-lasting increase in baseline sensory activity of the CB, a phenomenon termed as sensory long-term facilitation (sensory LTF).[17] In striking contrast, IH is ineffective in evoking sensory LTF in neonatal carotid bodies.[15] Third, IH has no significant effect on CB morphology in adult rats [17] whereas it caused hyperplasia of glomus cells in neonates.[16] Fourth, in adult rats, the augmented CB response to hypoxia is completely reversed after the cessation of IH [17] whereas the effects of neonatal IH persisted into adulthood.[15]

IH-exposed rat pups exhibit augmented hypoxic ventilatory response (HVR), a hallmark reflex response initiated by the CB.[12],[18] A similar increase in the HVR was also seen in preterm infants with recurrent apneas compared to infants without apneas.[10] The enhanced HVR evoked by neonatal IH, on the one hand, may be beneficial in the initial stages as it provides adequate oxygenation in infants with apnea, thereby preventing deleterious effects of hypoxia on the central nervous system. On the other hand, if the apneas persist, instead of being beneficial, the heightened hypoxic sensitivity of the CB may lead to breathing instability and increased incidence of apneas. Indeed, neonatal rats exposed to several days of IH exhibit greater number of apneas than control rat pups.[18],[19]


  Neonatal Intermittent Hypoxia Leads to Enhanced Catecholamine Secretion from Adrenal Medullary Chromaffin Cells Top


Souvannakitti et al. examined the effects of IH on CA secretion from AMC in neonatal rats in response to hypoxia.[20] CA secretion is monitored from dissociated chromaffin cells by carbon fiber amperometry. The number of chromaffin cells responding to hypoxia and the magnitude of CA secretion for a given level of hypoxia are greater in IH-exposed rats than the controls. The increased CA secretion by hypoxia is due to a greater number of secretory events as well as greater amount of CA released per se cretory event. IH increased both norepinephrine and epinephrine contents in neonatal adrenal medullae. In striking contrast, hypoxia-evoked CA secretion is reduced in rat pups exposed to continuous hypobaric hypoxia (0.4 ATM), suggesting that the augmented secretory response of AMC is unique to IH. Like CB, the enhanced AMC response to hypoxia is not reversed after the cessation of IH and persisted into adulthood.


  Physiological Consequences of Neonatal Intermittent Hypoxia in Adult Life Top


Adult rats that were exposed to IH in neonatal period showed (a) augmented CB and AMC responses to hypoxia, (b) enhanced HVR, a hallmark response of the carotid chemoreflex, (c) irregular breathing, (d) greater number of apneas, and (e) hypertension and elevated plasma CA levels as compared to control rats. These findings are reminiscent of recent clinical studies showing greater incidence of sleep disordered breathing with apnea [21],[22],[23] and hypertension in young adults and adults, respectively who were born preterm.[24]


  Reactive Oxygen Species: A major Cellular Mechanism Mediating the Effects of Neonatal Intermittent Hypoxia on the Carotid Body and Adrenal Medullary Chromaffin Cells Top


The above-outlined studies demonstrate that intermittent leads to augmented hypoxic sensing by the CB and enhanced CA secretion from AMC in neonatal rats, whereas these responses were not seen with continuous exposure to hypoxia. The major difference between intermittent and continuous hypoxia is the periodic oxygenation in the former but not the latter. In this respect, IH resembles ischemia-reperfusion. It is well known that during reperfusion, there is increased generation of reactive oxygen species (ROS).[25] The following observations demonstrate that ROS mediate the effects of neonatal IH on hypoxic sensing by the CB and AMC: (a) IH increased ROS levels in neonatal carotid bodies and adrenal medullae as evidenced by elevated malondialdehyde levels,[16],[26] which represent oxidized lipids and proteins,[27] and (b) antioxidant treatment prevented the augmented hypoxic response of the CB and AMC evoked by neonatal IH.[16],[20],[26] Interestingly, antioxidant treatment had no effect on hyperplasia of glomus cells by IH, suggesting that IH-induced augmented hypoxic sensitivity of the neonatal CB is not secondary to increased number of glomus cells.[16]

The elevated ROS levels by neonatal IH could be due to either increased ROS generation by pro-oxidant enzymes or decreased ROS degradation by antioxidant enzymes. The family of NADPH oxidases (Nox) constitutes one of the major sources of ROS in mammalian cells.[28] IH-exposed neonatal rat adrenal medulla show elevated levels of Nox2 and 4 mRNAs and increase in Nox enzyme activity.[26] On the other hand, mRNAs encoding antioxidant enzymes such as the manganese superoxide dismutase (Sod2), catalase 1, and glutathione peroxidase 1 were downregulated in IH-exposed neonatal rat carotid bodies and adrenal medullae.[19] These observations suggest that both decreased activity of antioxidant enzymes and increased activity of Nox contribute to elevated ROS levels by neonatal IH.


  Neonatal Intermittent Hypoxia Initiates Epigenetic Programming of the Redox State Top


Similar to cardiorespiratory changes, the increased ROS levels caused by neonatal IH were not normalized during room air recovery, rather persisted into adult life.[15],[20] A recent study examined the molecular mechanisms underlying the long-lasting effects of neonatal IH on ROS levels in the CB and adrenal medulla.[19] In this study, rat pups are exposed to IH from ages P0 to P10 and then reared under room air environment (normoxia) for 40 days. Analysis of mRNAs shows increased expression of genes encoding pro-oxidant enzymes and decreased expression of genes encoding antioxidant enzymes in carotid bodies and adrenal medullae of adult rats exposed to IH in the neonatal period as compared to controls.

Epigenetic mechanisms are heritable modifications of DNA and include DNA methylation and histone modifications. Epigenetic changes result in long-term alterations in gene expression.[29] Using the Sod2 as a model gene, Nanduri et al. showed that DNA hypermethylation contributes to neonatal IH-induced downregulation of Sod2 mRNA, protein, and the enzyme activity.[19] These authors further identified a single CpG dinucleotide within the Sod2 gene close to the transcription initiation site that was hypermethylated in response to neonatal IH. Neonatal rats exposed to IH were treated with decitabine, an inhibitor of DNA methylation. Decitabine treatment prevented DNA hypermethylation of the Sod2 gene and restored ROS levels to control values. Molecular mechanisms mediating the persistent upregulation of pro-oxidant enzymes by neonatal IH, however, remain to be elucidated. Notwithstanding these limitations, the study by Nanduri et al. demonstrate that neonatal IH initiates epigenetic changes that lead to long-lasting increase in ROS levels in the CB and adrenal medulla.[19]

Remarkably, decitabine treatment during the neonatal life prevented hypertension and increased apneas in adults.[19] These observations suggest that neonatal IH predisposes to cardiorespiratory dysfunction in early adulthood involving epigenetic regulation of the redox state.

Acknowledgment

This research is supported by National Institutes of Health grants HL-PO1-90554. We gratefully acknowledge the participation of Dr. Anita Pawar, Dangjai Souvannakitti, and Ying-Jie Peng in various experiments outlined in this article.

Financial support and sponsorship

This research is supported by National Institutes of Health grants HL-PO1-90554.

Conflicts of interest

There are no conflicts of interest.



 
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